Process for producing cobalt alloys

ABSTRACT

A process for producing oxygen-free cobalt based alloys having improved stability in high temperature irradiated sodium environments wherein cobalt is alloyed with chromium, tungsten, iron, carbon, nickel, molybdenum and titanium and thereafter the alloy is cold reduced 50 percent and heat annealed until it has a grain size of less than 10 microns.

United States Patent inventors Robert B. Herchenroeder Kokomo, Ind.; Joe G. Y. Chow, Northport; Albert 11. Fleitman, Smithtown, NY.

Appl. No. 15,237

Filed Feb. 27, 1970 Patented Nov. 16, 1971 Assignee The United States of America as represented by the United States Atomic Energy Commission PROCESS FOR PRODUCING COBALT ALLOYS 2 Claims, No Drawings u.s. Cl 148/2, 75/l7l,l48/l1.5

lnt.Cl c22r 1/10, C22cl9/00 FieldoiSearch 148/2,3,

Primary Examiner-Charles N. Lovell Attorney-Roland A. Anderson ABSTRACT: A process for producing oxygen-free cobalt based alloys having improved stability in high temperature irradiated sodium environments wherein cobalt is alloyed with chromium, tungsten, iron, carbon, nickel, molybdenum and titanium and thereafter the alloy is cold reduced 50 percent and heat annealed until it has a grain size of less than 10 microns.

PROCESS FOR PRODUCING COBALT ALLOYS PRIOR ART SUMMARY OF INVENTION A method of producing cobalt-based alloys, said alloys having stability in high temperature, radiation and sodium environment comprising charging a furnace with from 71.02 to 42.0 weight percent cobalt, based on the total weight of the alloy, from 24.0 to 19.03 weight percent chromium, based on the total weight of the alloy, from 21.84 to 4.92 weight percent nickel, based on the total weight of the alloy, from 16.0 to 4.50 weight percent of at least one member of the group consisting molybdenum and tungsten based on the total weight of the alloy, from 8.30 to 0.18 weight percent iron based on the total weight of the alloy, up to about 1.0 weight percent titanium based on the total weight of the alloy, about 0.03 weight percent carbon based on the total weight of the alloy, about 0.03 weight percent carbon based on the total weight of the alloy; heating the charge to a temperature ranging from about 2,500 F. to about 2,700 F. for a period ranging from about 15 to 20 minutes while maintaining the charge under a vacuum of less than three microns a mercury; adding an oxygen getter to remove any oxygen from the heated charge; and removing the oxygenated getter from the charge; cooling the heated charge to its freeze point and adding from 5.26 to 0.09 weight percent titanium based upon the total weight of the alloy; back-pressuring the chamber over the charge with an inert gas, e.g., argon; heating the charge containing the titanium under an atmosphere of inert gas to a temperature above 2,500" F. until a uniform melt is obtained; adding a sulfur getter to fix any sulfur contained in the charge, pouring the charge into a mold and cooling the cooling the poured charge until it is a solid billet; working the billet until it has a grain size of less than 10 microns by conventional means, e.g., reducing the billet thickness by 50 percent while maintaining the billet at a temperature below 2,100 F. and thereafter heating the reduced billet at a temperature of about 1 ,600 F. for about 16 hours.

DETAILED DESCRIPTION OF THE INVENTION The process of our invention can be carried .out using conventional alloying techniques and equipment. Care must be taken to eliminate oxygen as an impurity from the alloy. Thus, the alloy is heated under a vacuum of three microns or less until a homogeneous melt is obtained. Oxygen-getters are added to the melt to remove any oxygen impurities from the melt.

Our invention can utilize conventional oxygen-getters such as, but not limited to, aluminum, zirconium and magnesium and titanium. 1n the preferred embodiment of our invention, we employ aluminum as the oxygen-getter because it has a low vapor pressure, reacts quickly with the oxygen in the system and the aluminum oxide thus formed floats on top of the melt and adheres to the crucible and is thus readily removed with any other slag contained on the melt.

' The titanium in our alloy is added after oxygen impurities have been removed from the melt due to the high degree of reactivity between titanium and oxygen and the fact that only a small weight percent of titanium is desired in our alloying process.

After the melt is degassed, the vacuum chamber over the melt is backfilled with a partial pressure of an inert gas to assist in the retention of the high-vapor pressure sulfur getter, e.g., magnesium or mixtures of nickel and magnesium which is added to fix (tie up sulfur as a stable compound and prevent hot shortness) residual sulfur and to prevent undesirable meltcrucible reactions during the casting of the alloy.

After the alloys have been poured and cooled into a solid billet, we reduce (roll) the billet until its thickness has been reduced by about 50 percent. This reduction can be done at temperatures under 2,100 F. preferably is done at room temperature. Conventional milling equipment can be utilized to obtain the reduction.

The final annealing at a low temperature, e.g., 1,600 F. for a time period sufficient to cause recrystallization and secondary precipitation of particles is important as this annealing, together with the prior reduction produces a structure having a final grain size of less than 10 microns.

In selecting materials for the alloy, care should be taken to select materials which are substantially free of silicon, boron, and other impurities. Such impurities have a deleterious effect on the alloy performance in the higher temperature radiation sodium environment in which it is planned to employed the alloy.

Table I show several alloy samples which were made up in accordance with the practice of this invention. The figures given in this table are weight percentages based upon the total weight of the alloy.

These sample alloys were prepared as follows:

Raw materials -cobalt granuals, electrolytic nickel and lowimpurity iron, vacuum grade chromium, melting grade tungsten, molybdenum pellets, sponge titanium, high-carbon chromium and zirconium sponge.

Melting Sequence -A vacuum induction furnace was charged with cobalt, chromium, nickel, tungsten, iron, molybdenum, and high-carbon chrome containing carbon equal to about 0.025 weight percent of "the total charge. The vacuum chamber was then evacuated and the temperature slowly raised to 2,500 F. as measured by an optical pyrometer (because of the necessity of sighting through an extra glass in the vacuum shell, the optical pyometer readings are approximately F. less than the actual temperature. Therefore, a recorded optical pyrometer measurement of 2,500 F. would mean that the actual temperature of the melt was approximately 2,650 F.

After the cessation of the carbon boil and accompanying degassing, the temperature was reduced to the liquidus point of the melt and late additions of aluminum titanium, and powdered graphite were added and the temperature was again raised to 2,500 F. as measured by the optical pyrometer. At this point, the chamber was backfilled with argon to approxi' mately one-half atmosphere pressure and a quantity of magnesium, in the form of nickel-magnesiu, was plunged into the bath to fix the residual sulfur content. The melt was then cast into one nominally 15-pound round tapered ingot, one nominally 20-pound ingot and samples for chemistry and other analysis. The results of the chemistry analysis are shown in table I.

The ingots were the hot reduced (about 2,100 F.) in thickness to within about 30 percent of their final diameter and were coldswaged to their final size in the shape of rods and the billets were then heat annealed at a temperature of about 1,600 F. for 16 hours.

TABLE IL-EFFECTS F WORKED AND RECRYSTALLIZED A comparison of tables 11 and 111 discloses the beneficial results obtained when our novel alloying process is utilized to prepare alloys for use in high temperature irradiated environments.

We claim:

1. A method of producing cobalt based alloys, said alloys having stability in high temperature, radiation and sodium environment comprising;

a. charging a furnace with from 71.02 to 42.0 weight per- IRRADIATION ON COBALT BASE ALLOY COLD Yield Tensile Total Alloy Sample Irred. Test temp., strength, strength, elongation, No. fluence C. p.s.l. psi. percent A 0 760 31, 200 33, 000 57 0 800 500 19, 000 87 "1. 3X10" 650 80, 000 95, 000 29 "1. 3X10 750 32, 400 39, 800 24 "1. 3X10 800 26, 200 67 B 0 26 116, 000 179, 000 28 0 650 74, 600 82, 400 45 0 750 31, 700 33, 400 52 0 800 18, 500 800 63 1. 3X10 660 81, 200 97, 500 20 1. 3X10 760 41, 500 42, 600 22 C 0 117, 000 178, 000 15 0 650 72, 300 84, 500 34 0 760 32, 900 36, 000 47 0 800 20, 700 22, 300 71 1. 3X10 650 80, 500 94, 800 23. 6 1. 3x10 750 41, 600 44, 100 28 0 650 78, 800 92, 600 4 0 750 29. 300 32, 100 5 0 800 16, 700 19, 000 6 1. 3X10 650 79, 700 94, 800 2- 1. 3x10 750 40, 300 42, 700 3 1. 3x10 800 26, 400 28, 000 7 "E 0.82 Mev.

Norm: All specimens tested at a strain rate 0! 0.2% per min.

Table 111, shown below, shows the results of yield, tensile and elongation tests on cobalt alloys and on iron base alloy which were prepared by conventional techniques.

Table 111 Comparison of Properties of Iron and Cobalt Base Alloys (All specimens irradiated at less than 100 C. to 1.3)(10 E 0.82 MeV) Not Cold Worked and Recrystallized Test Yield Tensile Total Strain rate 2 percent per minute, all other specimens attained at 0.2 percent per minute. P8

cent cobalt, based upon the total weight of the alloy; from 24.0 to l9.03 weight percent chromium, based on the total weight of the alloy; from 2!.84 to 4.92 weight percent nickel based upon the total weight of the alloy; consisting of molybdenum and tungsten, based upon the total weight of the alloy; from 8.3 to 0.18 weight percent titanium, based on the total weight of the alloy; and about 0.03 weight percent carbon, based upon the total weight of the alloy;

. heating the charge to a temperature ranging from about 2,500 F. to about 2,700 F. for a period ranging from about 15 to about 20 minutes while maintaining the charge under a vacuum of less than 3 microns of mercury;

. adding an oxygen getter to the charge to remove any oxygen from the heated charge;

. cooling the heated charge to its freeze point and adding from 5.26 to 0.09 weight percent titanium based upon the total weight of the alloy;

. backpressuring the chamber over the charge with an inert heating the charge to a temperature above 2,500 F. until a unifonn melt is obtained and adding a sulfur getter in amounts sumcient to fix the residual sulfur;

. pouring the charge into a mold and cooling the poured charge until it is a solid billet;

. working the solid billet until it has a grain size of less than 10 microns by reducing the billet thickness about 50 percent while maintaining the billet at a temperature below 2,100 F. and thereafter heating the reduced billet at a temperature of about l,600 F. for about l6 hours.

2. A method in accordance with claim 1 wherein the solid billet is worked until it has a grain size of less than 3 microns.

i i t 

2. A method in accordance with claim 1 wherein the solid billet is worked until it has a grain size of less than 3 microns. 